Kraft pulping of hardwoods: Investigating the impact of wood microstructure on impregnation and delignification rate
Doctoral thesis, 2026
Kraft pulping accounts for most of the global paper pulp production. Nevertheless, the detailed mechanisms that govern delignification during this process are still not fully understood. Hence, this work investigated impregnation and kraft cooking of hardwoods, aiming to strengthen our knowledge regarding the rates and uniformity of lignin removal within wood chips, particularly considering the influence of specific morphological features.
The experiments included wood chips of alder, aspen, beech, and birch, and their behavior was examined from two perspectives: changes in global and local chemical composition, and changes in wood microstructure. The latter was evaluated in-situ via synchrotron X-ray tomography. Additionally, a multiscale model describing the delignification of birch chips during kraft cooking was developed.
The results revealed that the concentration and distribution of alkali within the chips after impregnation have major impact on delignification and are strongly affected by wood chip porosity. Furthermore, structural analysis during impregnation showed that vessels provide the main path for liquor penetration and distribution among adjacent cells. No substantial changes in cell wall thickness due to alkaline swelling were observed.
When comparing hardwood species, lignin removal was significantly faster in aspen. Delignification uniformity also increased with chip porosity and was shown to improve when utilizing low cooking temperatures (e.g., 145 °C) or impregnation liquors with high alkali concentrations. Differences in ray cells among the hardwoods had no clear impact on local rates of lignin removal. In terms of microstructural changes, delignification led to increased chip porosity and a minor decrease in cell wall thickness.
Finally, the proposed modeling approach has potential to be used for investigating the defibration point of wood chips. According to it, lignin removal from the cell walls appears to be limited by reaction kinetics and diffusion of lignin fragments, whereas the overall delignification behavior at the chip scale is heavily influenced by the balance between alkali transport and consumption.
The experiments included wood chips of alder, aspen, beech, and birch, and their behavior was examined from two perspectives: changes in global and local chemical composition, and changes in wood microstructure. The latter was evaluated in-situ via synchrotron X-ray tomography. Additionally, a multiscale model describing the delignification of birch chips during kraft cooking was developed.
The results revealed that the concentration and distribution of alkali within the chips after impregnation have major impact on delignification and are strongly affected by wood chip porosity. Furthermore, structural analysis during impregnation showed that vessels provide the main path for liquor penetration and distribution among adjacent cells. No substantial changes in cell wall thickness due to alkaline swelling were observed.
When comparing hardwood species, lignin removal was significantly faster in aspen. Delignification uniformity also increased with chip porosity and was shown to improve when utilizing low cooking temperatures (e.g., 145 °C) or impregnation liquors with high alkali concentrations. Differences in ray cells among the hardwoods had no clear impact on local rates of lignin removal. In terms of microstructural changes, delignification led to increased chip porosity and a minor decrease in cell wall thickness.
Finally, the proposed modeling approach has potential to be used for investigating the defibration point of wood chips. According to it, lignin removal from the cell walls appears to be limited by reaction kinetics and diffusion of lignin fragments, whereas the overall delignification behavior at the chip scale is heavily influenced by the balance between alkali transport and consumption.
in-situ tomography
impregnation
delignification
Modelling
hardwood
kraft pulping
Lecture hall HB3, Hörsalsvägen 10, Chalmers.
Opponent: Professor Christine Chirat, Université Grenoble Alpes, France.
Author
Carolina Marion de Godoy
Chalmers, Chemistry and Chemical Engineering, Chemical Technology
Kraft cooking of birch wood chips: Differences between the dissolved organic material in pore and bulk liquor
Holzforschung,;Vol. 77(2023)p. 598-609
Journal article
Kraft pulping of model wood chips: Local impact of process conditions on hardwood delignification and xylan retention
Holzforschung,;Vol. 78(2024)p. 446-458
Journal article
Marion de Godoy, C., Andersson, M., Hasani, M., Theliander, H., Uniformity of delignification during kraft pulping of hardwood chips: impact of wood structure and the importance of impregnation
Marion de Godoy, C., Laçaj, E., Hackenstrass, K., Galluccio, L., Yu, H., Florisson, S., Wohlert, M., Hall, S.A., Hasani, M., Theliander, H., Impregnation and delignification during kraft pulping of hardwood chips: characterization using in-situ X-ray tomography
Marion de Godoy, C., Kron, L., Hasani, M., Theliander, H., 1D Multiscale modeling of delignification rate in hardwood chips during kraft pulping
Peering inside wood chips: what can we learn about pulping?
Kraft pulp production is an essential industrial sector in modern society, providing raw materials for paper, packaging, textiles, hygiene products, and many other applications. In this process, wood chips are cooked with an alkaline liquor to remove lignin – a substance that acts as a glue between wood cells – and thereby liberate the cellulose fibers. However, the specific mechanisms governing this process are still difficult to pinpoint, as wood is an extremely complex material and many chemical reactions, physicochemical phenomena, and transport processes occur simultaneously during pulping.
In this context, this thesis aimed to shed light on how wood structure can affect lignin removal within wood chips of hardwoods (broad-leaved trees with diverse morphology). Local chemical analyses showed that it is easier to achieve a more uniform lignin removal in wood species with high porosity. This uniformity was linked to the initial composition and distribution of cooking chemicals within the wood chips and also depended on cooking temperature. In addition, experiments using in-situ X-ray tomography revealed how the kraft liquor penetrates wood and allowed the assessment of structural changes within wood chips while they were cooked. Lastly, a simple multiscale model was developed to describe lignin removal in the least accessible regions in wood chips, which can help future studies investigating fiber liberation.
Kraft pulp production is an essential industrial sector in modern society, providing raw materials for paper, packaging, textiles, hygiene products, and many other applications. In this process, wood chips are cooked with an alkaline liquor to remove lignin – a substance that acts as a glue between wood cells – and thereby liberate the cellulose fibers. However, the specific mechanisms governing this process are still difficult to pinpoint, as wood is an extremely complex material and many chemical reactions, physicochemical phenomena, and transport processes occur simultaneously during pulping.
In this context, this thesis aimed to shed light on how wood structure can affect lignin removal within wood chips of hardwoods (broad-leaved trees with diverse morphology). Local chemical analyses showed that it is easier to achieve a more uniform lignin removal in wood species with high porosity. This uniformity was linked to the initial composition and distribution of cooking chemicals within the wood chips and also depended on cooking temperature. In addition, experiments using in-situ X-ray tomography revealed how the kraft liquor penetrates wood and allowed the assessment of structural changes within wood chips while they were cooked. Lastly, a simple multiscale model was developed to describe lignin removal in the least accessible regions in wood chips, which can help future studies investigating fiber liberation.
Areas of Advance
Production
Subject Categories (SSIF 2025)
Wood Science
Paper, Pulp and Fiber Technology
DOI
10.63959/chalmers.dt/5847
ISBN
978-91-8103-390-8
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5847
Publisher
Chalmers
Lecture hall HB3, Hörsalsvägen 10, Chalmers.
Opponent: Professor Christine Chirat, Université Grenoble Alpes, France.